1. Field of the Invention
The present invention relates to a piezoelectric element and a process for producing the same, and more particularly to a piezoelectric element fit for use in converting electric energy to mechanical energy and vice versa in various kinds of equipment such as an actuator, a pressure sensor, a temperature sensor and so forth.
2. Description of Related Art
Previously, piezoelectric elements have been employed as vibrators, that is, as driving sources for discharging ink from ink-jet recording heads, for example. A piezoelectric element is generally structured so that the piezoelectric element has a poly-crystalline piezoelectric film, an upper electrode and a lower electrode with the piezoelectric film held between the electrodes. Platinum is used for the lower electrode, and titanium or a titanium oxide is used as an adhesion layer between the platinum and an underlying substrate.
The piezoelectric film is generally fabricated with Pb(Zr,Ti)O3 (hereinafter called xe2x80x9cPZTxe2x80x9d) as the principal component of a three-component system with a third component added to the two-component PZT. The piezoelectric film having such a composition may be formed through a sputtering, sol-gel, laser abrasion, or CVD method.
When the aforesaid piezoelectric element is applied to the actuator of an ink-jet recording head, a piezoelectric film (PZT film) is required to be about 0.5 xcexcm 20 xcexcm thick, for example, and is also required to have a high piezoelectric distortion constant. In order to obtain a piezoelectric film having such a high piezoelectric distortion constant, the film has to be annealed (heat-treated) at about 600xc2x0 C.-700xc2x0 C. to attain the growth of crystalline grains in the piezoelectric film.
When the PZT film having a thickness of about 0.5 xcexcm-20 xcexcm, for example, is formed by the sol-gel method, the sol of the PZT film is subjected to spin-coating for drying and pyrolyzing purposes in several cycles. Then, the products are subjected to pre-annealing as a first step. The sol is further subjected to spin-coating for drying and pyrolyzing purposes in several cycles, and then the products are subjected to annealing as a second step. Incidentally, the PZT film is usually formed by the use of sols having the same composition for the whole layer.
When a PZT film having the aforesaid thickness is formed, there develops a problem arising from a crack in the film obtained through the step of annealing the film after it is coated with the sol once. In order to solve this problem, the sol is subjected separately to spin-coating a plurality of times.
In the method of separately subjecting the sol to spin-coating a plurality of times, however, more lead ingredient is evaporated from the then uppermost layer at the time of pre-annealing and the then uppermost layer at the time of annealing than is evaporated from the multi-layer structure. The lead may also be diffused into the base (substrate). Both effects result in the decrease of the quantity of lead in the surface layer. For this reason, there develops a problem arising from a mismatching plane in the grain boundary of the layer formed at the time of pre-annealing the PZT film (hereinafter may be called the xe2x80x9clower layer) and the layer formed at the time of annealing (hereinafter may be called the xe2x80x9cupper layerxe2x80x9d). Moreover, another problem is that a decrease in the quantity of lead in the surface makes this surface portion have low-dielectric characteristics which tends to lower the piezoelectric characteristics.
The relation between x and y in a composition which is expressed by the general term AxByO3 for use in forming the piezoelectric film becomes y greater than x instead of x:y=1:1 as it should have been. This creates a problem because the B site becomes excessive and affects crystalline growth, and the piezoelectric characteristics are reduced as the crystal orientation is lowered.
To combat this problem, a method of excessively adding lead to the sol of the PZT film has been devised. However, the addition of excess lead decreases the withstand voltage of the film, which offsets any improvements in the piezoelectric characteristics.
An object of the present invention is to solve the foregoing problems by providing a piezoelectric element with improved piezoelectric and dielectric characteristics and a method of producing such a piezoelectric element which does not degrade the withstand voltage of the film.
In order to accomplish the object above, a piezoelectric element according to the present invention comprises a piezoelectric film, an upper electrode and a lower electrode with the piezoelectric film held between the electrodes is structured such that the piezoelectric film is formed through the steps of forming a film by applying a first sol for forming the piezoelectric film at least once, forming a film on the film coated with the first sol by applying a second sol having a greater lead content than the first sol, and subjecting both films to heat treatment at a predetermined temperature.
The piezoelectric film can further be built up by forming a film on the heat-treated film by applying the first sol at least once, forming a film on the film coated with the first sol by applying the second sol, and subjecting both the films to heat treatment at a predetermined temperature.
In the piezoelectric element thus formed, the ratio of lead in the thickness direction of the piezoelectric film is more uniform than the piezoelectric film of a conventional piezoelectric element. In other words, a difference in the existing quantity of lead along the thickness of the film is minimized, whereby both piezoelectric and dielectric characteristics are improved.
Further, a piezoelectric element comprising a piezoelectric film, an upper electrode and a lower electrode with the piezoelectric film held between the electrodes is such that the piezoelectric film is expressed by the general chemical formula AxByO3 (where, A=Pb, La or Ca or a combination thereof; and Bxe2x95x90Ti, Zr, Mg or Nb, or a combination thereof with a stoichiometric ratio of x to y expressed by x:y=1:1). When x=1 is set to 100%, the difference in the existing quantity of lead along the thickness of the film is 44% or less. Moreover, the piezoelectric film has a Perovskite structure.
The piezoelectric and dielectric characteristics are both improved as lead uniformly exists in the piezoelectric film as compared with the conventional film.
A process for producing a piezoelectric element comprising a piezoelectric film, an upper electrode and a lower electrode with the piezoelectric film held therebetween according to the present invention comprises the steps of first applying at least once, a first sol for use in forming the piezoelectric film on a substrate having the lower electrode formed thereon; second, applying a second sol having a greater lead content than the first sol; and third, subjecting the resulting films to heat treatment at a predetermined temperature after the second step.
In this process for producing a piezoelectric element, even though more of the lead component is evaporated from the layer formed with the second sol than any other layer or is caused to be diffused into the base (substrate) when the heat treatment is made at the third step, the content of lead in this layer is prevented from becoming smaller than that in the underlying structure because the content of lead has been increased therein beforehand.
A cycle comprising the first, second and third steps may be repeated at least once after the termination of the third step. Thus, the piezoelectric film is formed free from a crack, and a piezoelectric film having any desired thickness can be manufactured. With the above process, a mismatching plane is prevented from being formed on the grain boundary between the layers of films in each cycle.
The second sol may contain lead in greater amounts than the lead contained in the piezoelectric film which is desired and to be ultimately obtained. The lead quantity or the second sol should exceed the lead quantity of the desired piezoelectric film by not less than 5% and not greater than 43%.
Preferably, the second sol contains lead greater by not less than 20% and not greater than 30% than the lead contained in a piezoelectric film to be ultimately obtained.
A film formed by applying the second sol may be formed on the film formed by applying the first sol three times and both the films may be subjected to heat treatment at the predetermined temperature. Further, a film formed by applying the first sol three times may be formed on the heat-treated film and a film formed by applying the second sol may be formed thereon and then these films may be subjected to heat treatment at the predetermined temperature.
A cycle comprising the first, second and third steps may be repeated twice. Further, the second step may proceed after the first step is repeated three times.
A process for producing a piezoelectric element comprising a piezoelectric film, an upper electrode and a lower electrode with the piezoelectric film held between the electrodes comprises the steps of first applying, at least once, a first sol for use in forming the piezoelectric film on a substrate having the lower-electrode formed thereon; second, applying a second sol for use in forming the piezoelectric film; and third, subjecting the resulting films to heat treatment at a predetermined temperature after the second step, wherein the first and second sols have a composition capable or forming a piezoelectric film having the Perovskite crystalline structure and is expressed generally by AxByO3 and wherein the content of material constituting the A site of the first sol is greater than what constitutes the A site of the second sol.
In the piezoelectric film of the piezoelectric element thus manufactured through the aforesaid process, crystalline growth occurring at the time of heat treatment for crystallization is promoted. Therefore, orientation is increased and piezoelectric characteristics are improved.
The AxByO3 may comprise Pb, La or Ca or a combination thereof as A, and Ti, Zr, Mg or Nb or a combination thereof as B.
A partial component of the B site or its oxide may be used for the lower electrode as an adhesion layer with respect to the substrate.
The content of material constituting the A site of the first sol may exceed the content of material constituting the A site of the second sol, but the A site content of the first sol should be less than 1.4 times that of the second sol. An exemplary range for the A site of the first sol relative to the A site of the second sol is 1.2-1.3, inclusive.